1. Field of the Invention
The present invention relates generally to materials treatment methods, and particularly to a method of increasing the hardness of wurtzite crystalline materials. The method significantly increases the hardness of such materials so that cutting tools and inserts formed from the hardened wurtzite crystalline material have significantly increased longevity in interrupted and ultra-precision cutting.
2. Description of the Related Art
Extremely hard materials have long been recognized as optimal for forming cutting tools, tool inserts, drill bits and inserts for such bits, and related components. Generally speaking, the harder the tool or insert, the greater its longevity before requiring sharpening or replacement. This is dependent upon many different factors, such as the material of which the workpiece is formed, the specific geometry of the cutting tool or insert, the cutting depth and angle relative to the workpiece, the cutting speed and lubricant (if any), and other factors.
Diamond has long been recognized as the essentially universal standard of hardness. Monocrystalline cubic structure diamonds have a hardness on the order of 100 GPa (gigaPascals), or about 14,500,000 psi (pounds per square inch) using the Vickers hardness test. While all diamond is formed of essentially pure carbon, the crystalline structure may differ. Another allotrope of diamond is known as lonsdaleite, which has a hexagonal crystal structure that approaches the hardness of cubic crystalline diamond in its natural form. However, a simulated pure sample of lonsdaleite has been found to have a hardness on the order of half again that of cubic crystalline diamond. Lonsdaleite is very rare, being formed only as a result of some meteor impacts and by synthesis.
While diamond material is considered to have the optimal hardness for use on the cutting edges of tool bits, cutting inserts, and the like, it does have its drawbacks, in that is not as chemically and thermally stable as many other materials that may be used for such purposes. While the use of diamond (both natural and synthetic) to form the cutting edges of various tool bits and cutting inserts is well known, there is nevertheless considerable interest in finding even harder materials and/or materials having greater chemical and thermal stability to provide greater economy and greater longevity for such tool bits and inserts.
Crystalline boron nitride (BN) is one such material. BN is not found in nature, but must be synthesized. However, crystalline BN can vary considerably in hardness, primarily depending upon the specific crystal matrix or structure of which the compound is formed. For example, BN having a cubic crystal structure (cBN) may have a hardness varying from 30 GPa to 43 GPa, depending upon the orientation of the applied test force relative to the crystal structure.
Another form of crystalline boron nitride may be formed from the shock compression of amorphous boron nitride, or treating hexagonal boron nitride (hBN) under extremely high static pressure and temperature. Such treatments produce boron nitride having a metastable crystal structure. This crystal structure is known as wurtzite boron nitride, or wBN, to differentiate it from cubic boron nitride, or cBN. Further shocks or treating with higher pressure and temperatures will transform wBN to cBN. The hardness of wBN has been determined in a range from 223 GPa to 52.5 GPa by the Vickers hardness test. Pure wBN, or wBN mixed with cBN in a polycrystalline matrix in a binder, results in an extremely hard material second only to diamond in hardness. The advantages of such wBN and/or cBN material in terms of thermal and chemical stability and resulting longevity make it attractive, even though its absolute hardness is less than diamond.
Thus, a method of increasing the hardness of wurtzite crystalline materials solving the aforementioned problems is desired.